1. Field of Invention
The present invention relates in general to free-space, diffraction gratings, and more particularly to free-space, diffraction gratings having a high throughput efficiency.
2. Discussion of Related Art
Diffractive elements are used in many application areas to spatially separate the component wavelengths of polychromatic light. For example, in telecommunications applications, diffractive elements may be used to spatially separate the channels of a wavelength division multiplexed (WDM) signal for subsequent processing (e.g., gain equalization or switching). Examples of diffractive elements for achieving spatial separation, include fiber Bragg gratings (FBG), and reflective and transmissive free-space diffraction gratings. Each of these diffractive elements offers performance advantages and disadvantages. A diffractive element's suitability for a specific application is dependent on characteristics such as throughput efficiency, dispersion, polarization-dependent loss (PDL), as well as the diffrative element's size, manufacturability, expense, and sensitivity to environmental conditions. Free-space diffraction gratings are referred to herein below as diffraction gratings or simply as gratings.
As is well known, reflective diffraction gratings are typically comprised of a reflective surface having a plurality of spatially-separated, reflective ridges and intervening grooves that induce a spatial, periodic phase delay on a wavefront of a beam of light incident thereon. The phase delay causes diffraction of the beam of light and angularly separates the beam into its component wavelengths of light. Conventional reflective, free-space diffraction gratings offer unique characteristics that make them suitable for many applications. However, as discussed below, they have performance shortcomings.
Also as is well known, transmissive diffraction gratings are typically comprised of a transmissive substrate having a plurality of spatially-separated, transmissive ridges and intervening grooves that induce a spatial, periodic phase delay on a wavefront of a beam of light. The phase delay causes diffraction of the beam of light and angularly separates the beam into its component wavelengths of light. Similar to conventional reflective diffraction grating, conventional free-space transmissive diffraction gratings offer unique characteristics that make them suitable for many applications. However, as discussed below, they have performance shortcomings.
The term “throughput efficiency” is defined herein to mean the logarithm of the ratio of the useable portion of the output optical power to the total optical power incident on the diffraction grating. In many applications, the useable portion of the output power corresponds to a single diffraction order. This diffraction order is referred to herein as the diffraction order of interest. A high throughput efficiency typically means greater than approximately 85%.
In telecommunications applications, the use of optical processing elements, such as gratings, having higher throughput efficiencies often results in a reduced need for amplifiers to boost a transmitted signal. Further, in many systems, a signal is first demultiplexed by a grating and then re-multiplexed by the grating. In such systems, the desirability of gratings having high throughput efficiency is compounded by the fact that the effects of low throughput efficiency are doubled.
The term “Polarization Dependent Loss” (PDL) is defined herein to mean the logarithm of the ratio of the throughput efficiency of the TE-polarized portion light (expressed in power) to the throughput efficiency of the TM-polarized light (expressed in power). In many systems, unpolarized light is incident upon a grating. In such systems it is typically desirable that the transmission of light by the system be independent of polarization so as to avoid producing a polarized output (i.e., it is desirable that the PDL be as near to zero as possible). However, conventional diffraction gratings typically transmit light in a highly polarization-dependent manner, particularly when they provide a high dispersion.
The term “angular dispersion” is defined herein to mean a variation of the wavelength of light as a function of angle. For example, processing of a polychromatic signal by a diffraction grating gives rise to a corresponding dispersion. Dispersion by free-space reflective diffraction gratings may be characterized using the well known grating equation,
                              sin          ⁡                      (                          θ              m                        )                          =                              sin            ⁢                                                  ⁢                          (                              θ                i                            )                                +                      m            ⁢                          λ              Λ                                                          (        1        )            
where θi is the angle of the incident beam with respect to the grating normal, m indicates a diffractive order, θm is the angle of the mth diffractive order with respect to the grating normal, λ is the wavelength of the light, and Λ is the period of the grating.
More particularly, for a plurality of wavelengths incident on a diffraction grating at a single angle, angular dispersion is given by the equation,
                                          ⅆ                          θ              m                                            ⅆ            λ                          =                  m                      Λ            ⁢                                                  ⁢            cos            ⁢                                                  ⁢                          θ              m                                                          (        2        )            
The term “linear dispersion” is defined as the product of angular dispersion and a selected length. Devices that process the spatially-separated channels of a WDM signal typically require a specific linear dispersion. Accordingly, it is typically desirable to have angular dispersion be as large as possible to minimize the length necessary to achieve a given amount of linear separation. As used herein below, the term “dispersion” used alone refers to angular dispersion.
For example, conventional design techniques may be used to produce diffraction gratings selected to provide a suitable throughput efficiency, dispersion, and PDL for a particular application. However, in many instances, a conventional grating made according to those techniques is not capable of producing suitable amounts of each. For example, a free-space metal reflective grating designed to have a grating frequency of 1800 lines/mm is known to have a 90% throughput efficiency for TM-polarized light in a wavelength range of 0.8 to 0.9 μm. However, the efficiency of the TE-polarized light in this wavelength range is below 50%.
A free-space metal reflective diffraction gratings designed to have a 1800 lines/mm grating frequency also provide high throughput efficiency for TE-polarized light in a very narrow band around λ=0.5 μm; however, the throughput efficiency for TM-polarized light in this wavelength range is below 50% (see pg. 78 of Diffraction Gratings and Applications, by Loewen, published by Marcel and Dekker, 1997). Additionally, conventional free-space gratings may be selected to provide high efficiency for both TE-polarized and TM-polarized light in a selected wavelength range; however, the dispersion in the selected range is low.
While reflective and transmissive gratings designed using conventional techniques may provide appropriate throughput efficiency, dispersion or PDL for some applications, there remains a need for grating elements providing appropriate combinations of each characteristic, as well as having an appropriate size, manufacturability, and expense.